It looks like Genesis has either developed finrot on just his bottom fin, or he's started biting it. But he's also got a rip going, so I don't know if he overflared (he's been flaring a lot lately) or what.
The black edge on his bottom fin is missing, though. So I thought he may have bitten it rather than developed rot.

Biting their tails, or ripping the fins can turn into fin rot because the fins are damaged already, use some aquarium salt and if it is fin rot it should start getting better (hopefully you've caught it early) and if it's tail biting, the salt should help to keep it from getting infected. Watch your guy carefully for a while(long enough that he stops paying attention to you) and see if you is biting his fins at all, it may take a few trys before you actually see him nipping at himself.

My fish Flippy managed to bite his top dorsal fin somehow...along with the caudal and anal fin. Keep his water really clean and it should heal up if he doesn't bite it again. If his fin looks like it is shrinking instead of having a chunk missing then it may be finrot. If theres a rip check your aquarium decor to make sure nothing is snagging his fins...You can check by using a pair of nylon hose and rub the decor against it..If it snags then it's no good for betta fins.Sometimes bettas can over-flare and rip a fin though..I had my one betta do that once..He is a VT. He ended up looking like a double tail. haha. If that is the case then make sure his water is clean and it should heal up nicely.